The present invention relates generally to the field of overcurrent protection circuits and in particular to overcurrent protection circuits including an auto-reset breaker and PTC resistor(s).
Many automotive-related electrical circuits involve low voltage (e.g., 12 VDC) and relatively high currents (e.g., 10–250 A). Examples include the starter motor in automobiles and trucks, as well as aftermarket accessories for automobiles, ATVs and off-road vehicles, such as winches. Automatically resetting thermal breakers are widely used for protection in these applications. These auto-reset breakers allow for brief or intermittent high currents, but will trip, or cause an open circuit and thus prohibit current flow, under sustained high current. The breakers are thermally actuated.
A representative auto-reset thermal breaker comprises a bi-metal element and a pair of contacts. The bi-metal element changes shape when heated, breaking the contacts and opening the circuit. A snap action is accomplished by forming the bi-metal element into a shape that inverts as a reaction to reaching a design temperature. Once the circuit opens, the bi-metal element begins to cool and returns (snaps) back to the closed position, allowing current to flow. This is the auto-reset feature.
The breaker trip curve is based on a time/temperature relationship. Breakers are rated for the maximum sustained current that they will allow to flow continuously without tripping. That is, the breaker rated current is the maximum “never trip” current value. A characteristic of auto-reset thermal breakers is allowing brief or intermittent currents far in excess of their rated capacity, tripping only when the current generates sufficient heat.
Since intermittent use of aftermarket products is expected, these products are sold with performance claims that exceed actual performance available under sustained conditions. That is, the claims assume a duty-cycle for the products' use. The manufacturers use under-rated auto-reset breakers for fault protection. The breakers are sized to allow for claims of product performance and protection of the device that are only true under the assumed duty cycle. For example, the manufacturer of a bumper-mounted winch may assume a usage duty cycle of thirty seconds use during any four minute period. The manufacturer may then choose an auto-reset breaker that will not heat up sufficiently under the current load to trip within thirty seconds, although it will trip if the current load is sustained much longer than thirty seconds (e.g., the current load exceeds the rating of the breaker). The three and a half minutes of assumed non-use is sufficient time for the breaker to cool, so as to not trip during another thirty seconds of use. This duty cycle may be typical of a one using the winch to pull a truck out of the mud, and the typical user will experience the performance claimed by the manufacturer.
One using his winch all day to pull stumps from the ground, however, will exceed the designed duty cycle, and will experience repeated tripping of the breaker. During such sustained high-current use—which also occurs under a short-circuit condition—the auto-reset breaker will continue resetting, continuing to allow intermittent high currents in the system. These repeated high-current conditions could cause wiring and other system components to heat up to dangerous levels, creating a fire hazard.
Unanticipated, sustained low-current use may also create a fire hazard. For example, the starter motor in an automobile may draw up to 250 A on a cold morning; and 45–50 A during more temperate weather. This circuit may be protected by a 280 A fuse. The current draw is designed to be brief—on the order of a few seconds for a well-tuned engine. However, if some fault failed to disengage the starter motor after the engine started, it may continue to draw, e.g., 50 A, well below the current that would blow a 280 A fuse. A sustained current of 50 A may cause the wiring or components—designed only for brief currents—to heat to dangerous levels, presenting a potential fire hazard.
Protective devices other than auto-reset breakers are rarely used in aftermarket products due to adverse customer perception. For example, fuses are never used because blowing a fuse prevents use of the product until the fuse is changed, and leads to customer perception of inferior quality. Manual reset breakers are sometimes used but are considered a nuisance by customers, and are often defeated to prevent them from tripping.
Hence a need exists in the art for an overcurrent protection circuit that allows for the industry practice of duty-cycle based performance claims, and additionally provides effective protection against high-current thermal damage. In addition, a need exists for an overcurrent protection circuit that protects against both very high excess current, and unanticipated sustained low current.